Set up HTML::Proofer

FYI this won't pass until #102 is merged, so wait until then.  Did the merge in a throwaway branch just to be sure:

![screen shot 2013-10-23 at 12 20 03 am](https://f.cloud.github.com/assets/86842/1388120/63513cda-3bb4-11e3-8eb1-e5d3d9da2e8d.png)

/cc @gjtorikian